Sorry for necro-ing this thread, but I hope this might be handy for anyone trying to safe a hasMany relation. I have made some adjustments to the code of @olivierpons. His code didn't work for me and I do not want to check for modified records but want to send everything. Code below was tested against 4.1.1

Have you explored the source? If you step through the call through sync() you can most likely see how it is determined which records are dirty. If you can override this method with an extra hook to check for changed parts your done.

But I cant find a way to use autostore on the master models store, so I only need to modfiy child model data. Have you another hint for me?
I find a function "afterEdit" on the model which fires the "afterEdit" event on the associated store, but this needs to bubble up to the master model store. I dont know how this can be done.